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kptr_restrict 向用户空间内核中的指针(/proc/kallsyms-modules显示value全部为0)	`LinuxDeviceDrivers/study/debug/filesystem/procfs/kptr_restrict`

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#1 /proc/kallsyms显示value全部为0

今天一个同事问我 cat /proc/kallsyms 显示 value 全部为 0. 我在手机端试了一下, 果然如此.

切换到 root 用户运行, 依然是 0. 感到十分奇怪, 因为内核发生 crash 或者打开 trace 的时候, 都是调用的 sprint_ symbol 来打印的. 为啥内核可以, 用户态 cat 就不行呢?

后来发现是系统为了保护这些符号地址泄露, 而用的一种保护手段, 从而使除 root 用户外的普通用户不能直接查看符号地址.

#2 kptr_restrict 介绍

原因在于内核文件 kallsyms.c 中的显示符号地址命令中做了如下限制.

seq_printf(m, "%pK %c %s\n", (void *)iter->value, iter->type, iter->name);

只需要把其中的 %pK 换成 %p 就可以让普通用户查看符号地址了. 很多提权漏洞一般会用到此处的修改来获取符号地址

内核提供控制变量 /proc/sys/kernel/kptr_restrict 来进行修改. 从内核文档 Documentation/sysctl/kernel.txt 中可以看到 kptr_restrict 用于控制内核的一些输出打印.

Documentation/printk-formats.txt 有更加详细的描述, 除了我们平时遇到的一些打印格式之外, 还有一些比较特殊的格式(我以前没注意到).

==============================================================

kptr_restrict:

This toggle indicates whether restrictions are placed on
exposing kernel addresses via /proc and other interfaces.

When kptr_restrict is set to (0), the default, there are no restrictions.

When kptr_restrict is set to (1), kernel pointers printed using the %pK
format specifier will be replaced with 0's unless the user has CAP_SYSLOG
and effective user and group ids are equal to the real ids. This is
because %pK checks are done at read() time rather than open() time, so
if permissions are elevated between the open() and the read() (e.g via
a setuid binary) then %pK will not leak kernel pointers to unprivileged
users. Note, this is a temporary solution only. The correct long-term
solution is to do the permission checks at open() time. Consider removing
world read permissions from files that use %pK, and using dmesg_restrict
to protect against uses of %pK in dmesg(8) if leaking kernel pointer
values to unprivileged users is a concern.

When kptr_restrict is set to (2), kernel pointers printed using
%pK will be replaced with 0's regardless of privileges.
==============================================================

kptr_restrict	权限描述
0	root和普通用户都可以读取
1	root用户有权限读取, 普通用户没有权限
2	内核将符号地址打印为全0, root和普通用户都没有权限

kptr_restrict 值为 2 时, 所有用户都无法读取内核符号地址.

kptr_restrict 值为 1 时, 普通用户都无法读取内核符号地址, root 用户可以查看.

kptr_restrict 值为 0 时, 所有用户都可以读取内核地址.

注意 kptr_restrict 对内核中很多地址和符号表的信息导出都有影响, 比如 /proc/modules 等.

#3 kptr_restrict的设计

kptr_restrict 在内核 commit 455cd5ab--kptr_restrict for hiding kernel pointers from unprivileged users, 具体实现源码位于 lib/vsprintf.c, line 1708

#http://elixir.free-electrons.com/linux/v4.13.9/source/lib/vsprintf.c#L1708
	case 'K':
		switch (kptr_restrict) {
		case 0:
			/* Always print %pK values */
			break;
		case 1: {
			const struct cred *cred;

			/*
			 * kptr_restrict==1 cannot be used in IRQ context
			 * because its test for CAP_SYSLOG would be meaningless.
			 */
			if (in_irq() || in_serving_softirq() || in_nmi()) {
				if (spec.field_width == -1)
					spec.field_width = default_width;
				return string(buf, end, "pK-error", spec);
			}

			/*
			 * Only print the real pointer value if the current
			 * process has CAP_SYSLOG and is running with the
			 * same credentials it started with. This is because
			 * access to files is checked at open() time, but %pK
			 * checks permission at read() time. We don't want to
			 * leak pointer values if a binary opens a file using
			 * %pK and then elevates privileges before reading it.
			 */
			cred = current_cred();
			if (!has_capability_noaudit(current, CAP_SYSLOG) ||
			    !uid_eq(cred->euid, cred->uid) ||
			    !gid_eq(cred->egid, cred->gid))
				ptr = NULL;
			break;
		}
		case 2:
		default:
			/* Always print 0's for %pK */
			ptr = NULL;
			break;
		}
		break;

#4 参考

Introducing Linux Kernel Symbols

Is there a way to set kptr_restrict to 0?

kptr_restrict for hiding kernel pointers from unprivileged users

Linux kallsyms 机制分析

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